This invention relates to the technology of non-stoichiometric crystalline compounds and the method for their purification by simultaneously eliminating residual impurities and controlling the composition of non-stoichiometric compounds possessing high vapor pressures at and below their melting points and the construction of an apparatus therefor.
The technological processes developed for the production of semiconductor materials now used for the fabrication of semiconductor devices (silica, germanium, gallium arsenide, etc.) such as zone melting and growing single crystals from the melt cannot be used for the processing of non-stoichiometric compounds possessing high vapor tensions well below their melting points, such as the semiconductor compounds of general formulas A.sup.II B.sup.VI, A.sup.IV B.sup.VI, etc. Although the method described here uses the well-known non-stoichiometric compound A.sup.II B.sup.VI by way of example, it can be used for the purification of very different compounds possessing high vapor tension in the solid state including crystalline stoichiometric organic compounds.
A crystalline compound A.sub.n B.sub.m is stoichiometric only if, upon sublimation, the vapor molecules do not dissociate, i.e., if the sublimation process can be described using the notation introduced by Kroger by the reaction EQU (n A.sub.A +m B.sub.B).sub.S .fwdarw.A.sub.n B.sub.m (v) (1)
Applying the mass action law, we have EQU P.sub.AnBm =K(T) (2)
In this case, the compound behaves like any elemental crystal, and it is impossible to add to (or to remove from) the compound atoms of its constituents in a proportion different from the ratio of the number of sites normally occupied in the crystal lattice of the stoichiometric compound.
Most generally the vapor phase resulting from the sublimation of crystalline compounds consists of the products of the dissociation of the molecules A.sub.n B.sub.m and the process of sublimation is described by a reaction such as EQU n A*.sub.A +m B*.sub.B .fwdarw.n A(v)+m B(v) (3) EQU P.sub.A.sup.n .multidot.P.sub.B.sup.m =K.sub.2 (T) (4)
For the compounds A.sup.II B.sup.VI we have EQU P.sub.A .multidot.P.sub.B.sup.1/2 =K.sub.2 (T) (5)
as the vapor phase contains atoms A and molecules B.sub.2.
In this case the composition of the crystal A.sub.n B.sub.m can be modified by heating it in a closed space where a definite pressure of one of its constituents is established. The ratio of sites occupied by the atoms in the crystal lattice can be modified by diffusing in it vacancies or interstitial atoms. By means of this procedure it is possible, in principle, to obtain a given non-stoichiometric composition (i.e., at a given concentration of point defects). However, this is by no means practical, as the process of diffusion proceeds very slowly and cannot eliminate the residual impurities always present in any crystal.
As the vapor phase of practically all inorganic compounds A.sub.n B.sub.m always contains products of the dissociation of the molecules A.sub.n B.sub.m, the problem of controlling their properties is that of being able to purify them and of achieving precision-control of their composition. The strictly stoichiometric composition could be attained only if the energies of formation of the point defects on both of the sub-lattices A and B were equal. As a rule, however, they are different.
The necessity to control precisely the concentration of point defects follows from the fact that in these compounds the point defects do ionize like the impurities incorporated in their lattices. Consequently, the physical and physicochemical properties of the semiconductor compounds strongly depend upon the concentrations of the point defects and of the residual impurities. In a typical compound A.sup.II B.sup.VI, the extent of the domain of existence is of the order of 10.sup.-3 -10.sup.-2 atomic percents and the concentration of ionized point defects may be 10.sup.19 /cm.sup.3. The partial vapor pressures of the constituents depend largely on the concentration of the point defects and may vary by a factor of 10.sup.6. This latter point is crucial for the technological processes of purification or crystal growing.
Whatever the purity of the constituents and the procedure of synthesis, it is practically impossible to insure the reproduction of the composition and hence that of the properties. At any temperature there are only three compositions of the crystalline compounds that can be precisely reproduced. Two are those that correspond on the T-x phase diagram to the two borderlines of the existence domain. The third is that which corresponds on the P-x diagram to the composition of minimum pressure. Compositions of the first and second type result in the greatest concentrations of point defects at the temperature considered; and when ionized (at least at room temperature and above) the point defects determine the type and the value of the electrical conductivity and have the greatest ratio of partial pressures of the constituents.
A crystalline compound having the composition of the point P.sub.min at a given temperature is characterized by the fact that the equilibrium compositions of the solid and vapor phases are equal (congruent sublimation). The composition of the point P.sub.min does not coincide with stoichiometry as the smallest difference between the energies of formation of the defects on the sub-lattices A and B lead to a displacement of the minimum of free energy of the phases in equilibrium towards the side where the energy of formation is the least. By heating a compound of arbitrary composition under conditions such that the vapor freely leaves the surface of the solid, the composition of the compound crystallizing at a lower temperature will be richer in constituent B with lower energy of formation, i.e., nearer to the point P.sub.min, and the vapor richer in constituent A, as the free energy correspondingly diminishes. The minimum of the total pressure P=P.sub.A +P.sub.B2 is observed when ##EQU1##
The position of the point P.sub.min and its displacement with temperature are dependent upon the nature of the point defects and the presence of residual and doping impurities.
Consequently, to produce materials with definite properties, it is necessary to have a technological process that can bring the composition (i.e., the concentration of point defects) of the compounds to that corresponding to P.sub.min and to purify them (i.e., to remove most of the impurities) at the same time.
The present invention concerns the construction of an apparatus realizing simultaneously the large-scale elimination (to a level below 10.sup.-5 -10.sup.-6 atomic %) of the impurities and the reproducible purification of non-stoichiometric compounds to approach the composition defined as the point P.sub.min. The method of purification described below can be used to process materials on the kilogram scale.